Embodiments of the disclosed technology relate to a pixel unit circuit, a pixel array, a display panel and display panel driving method.
As a light emitting device of current type, an organic light emitting diode (OLED) has been increasingly used in high-performance display apparatuses. A traditional passive matrix organic light emitting display (PMOLED) requires less driving time for a single pixel as the size of the display is gradually increased, thus it is necessary to increase transient current and increase power consumption. Meanwhile, application of a large current may cause the voltage drop on the ITO (indium tin oxide) line to an extremely large level and make the operation voltage for the OLED extremely high, and thereby the efficiency thereof is reduced. By contrast, an active matrix organic light emitting display (AMOLED) can input current for each pixel in a line by line scan manner with a switching element, and can solve these problems.
During the operation of the AMOLED pixel circuit, due to the uniformity of the threshold voltage of the TFTs as switching elements, the uniformity of the OLED itself or resistance voltage drop (IR Drop, a phenomenon in which, in a rear board, the voltage of a region that is close to the ARVDD power supply position is higher than that of a region that is far away from the power supply position) etc, circuit instability and unevenness of the OLED luminance may be incurred, thereby the pixel circuit array as a whole is affected. Therefore, the circuit being driven by the OLED needs to be improved in related arts, so that compensation is performed on the pixels with the OLED driving circuit.
According to driving type, the AMOLED can be divided into three categories, i.e., digital type, current type and voltage type. Similar to the traditional AMOLED driving method, the driving method of the voltage type is a method in which a voltage signal representing a gray scale is provided by an integrated driving chip, and the voltage signal will be converted to a current signal inside the pixel circuit so as to drive the OLED pixel. This method is advantageous in that the driving speed is fast and the implementation is easy, is suitable to driving display panels of a large size, and has been widely employed in industries.
FIG. 1 shows a first kind of driving circuit of the voltage type for driving the OLED in related arts. In each pixel, a voltage signal on the data line is transmitted by the T2 to the gate of the T1, and the received data voltage signal is converted by the T1 into a corresponding data current signal and supplied to the OLED. When normally operated, the T1 is in a saturation condition, and the current thereof can be represented as:
                              I          OLED                =                              1            2                    ⁢                                    μ              P                        ·            Cox            ·                          W              L                        ·                                          (                                  Vdata                  -                  ARVDD                  -                  Vthp                                )                            2                                                          (        1        )            where μp represents a mobility of carries, Cox represents a gate oxide capacitance, W/L represents a ratio of width to length of a TFT channel, Vdata represents a data voltage, ARVDD represents a power supply of the rear board of the AMOLED and is shared by all of the pixel unit circuits, and Vthp represents the threshold voltage of the T1. As can be known from the above expression, if the Vthp of the driving TFT (T1 in FIG. 1) in different pixel unit circuits are different, there exists a difference in the currents delivered to the respective OLEDs even if the data voltages to be delivered are of the same; meanwhile, if the ARVDD practically applied to the respective pixels are different, there also exists a difference in the currents delivered to the OLEDs.
FIG. 2A is a schematic diagram showing a second kind of the driving circuit of the voltage type for driving the OLED in related arts, and FIG. 2B shows a timing control diagram for the driving circuit of the voltage type. In this circuit, the voltage applied to the gate of the T2 is a voltage of VDATA+Vthp, which is independent of the power supply voltage VDD, thus this circuit can compensate for the IR Drop, but can not compensate for the uniformity of the TFTs.
FIG. 3A is a schematic diagram showing a third kind of the driving circuit of the voltage type for driving the OLED in related arts, and FIG. 3B shows a timing control diagram for the driving circuit of the voltage type. In this circuit structure, the voltage practically applied to the gate of the transistor T1 is independent of the threshold voltage Vth of the T1 and the power supply voltage ELVDD, and the threshold voltage uniformity of the driving transistor T1 and the IR Drop can be compensated for. However, this circuit requires four TFTs and two capacitors, and the voltage practically applied to the gate of the transistor T1 is associated with a ratio of the two capacitors; whereas the magnitudes of the two capacitors in this circuit differ not much, and a dynamic range of the inputted voltage is relatively small.
FIG. 4A is a schematic diagram showing a fourth kind of the driving circuit of the voltage type for driving the OLED in related arts, and FIG. 4B shows a timing control diagram for the driving circuit of the voltage type. In this circuit, the current inputted to the OLED remains constant, and the uniformity of the OLED can be compensated for; however, the voltage applied to the gate of the transistor T1 is associated with both of the threshold voltage Vth of the T1 and the power supply voltage ELVDD, and the threshold voltage uniformity of the driving transistor T1 and the IR Drop can not be compensated for.